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Subaru EJ Series Engine Compression Ratio Calculations


This is a brief CR FAQ regarding the EJ series engines used on various Subarus sold into the North American market. I will start with engines that I have personal experience with, and may be expanded to other EJ series motors as information becomes available.

Q: What are the differences between the 2.2L and 2.5L engines?
A: This is definitely a frequently-asked question. One of the best places to start with answers is the NASIOC FAQ's.

Here is a summary of commonly-debated numbers in the NASIOC forums:

Model Years Engine Applications Displacement (cc) Published/Estimated Compression Ratio
1990-1996 2.2L SOHC Legacy, Impreza 2212 9.5:1
1991-1994 2.2L SOHC Turbo Legacy Turbo 2212 8.0:1
1993-1996 1.8L SOHC Impreza 1820 9.5:1
1997-1998 2.2L SOHC Legacy, Impreza 2212 9.7:1
1998 2.5L DOHC Legacy, Impreza 2457 9.7:1
1999 2.5L SOHC Legacy, Impreza, Forester 2457 Range from 9.7:1 to 10.0:1
2000-2004 2.5L SOHC Legacy, Impreza, Forester 2457 10.0:1
2002-2004 2.0L DOHC Turbo Legacy, Impreza, Forester 1994 8.0:1
Q: Didn't Subaru redesign the engine over the years and change compression ratios?
A: First, see the answers to the previous question. Most current information available is from numbers published by Subaru, or from heresay. It is the latter that has prompted me to take measurements to confirm the numbers.

Q: What are the piston and cylinder volumes? Has anyone cc'd these engines?
A: The NASIOC archives (look for the "search" link) are usually a good place to find this type of information, but I have not been successful from searching the archives. There are plenty of questions, but not many answers. I'm certain that some tuning shops have this information, but I have not seen their data.

This table summarizes some measurements and calculations for stock configurations:

Engine Block Cylinder Head Bore¹ (cm) Stroke¹ (cm) Displ (cc) Gasket bore¹ (cm) Gasket Thickness (cm) Quench Vol (cc) Head Vol (cc) Piston Vol (cc) Calc'd CR Notes
MY90 EJ22 MY90 EJ22 9.69 7.5 553 9.69 0.152 11.2 40* 14.5¤ 9.4:1 EJ22 9.5:1 stock
MY91 EJ22T MY91 EJ22T 9.69 7.5 553 9.69 0.152 11.2 40* 28²¤ 8.0:1 EJ22T 8.0:1 stock
MY95 EJ18 MY95 EJ18 8.79 7.5 455 8.79 0.152 9.2 TBD 14 TBD EJ18 9.5:1 stock
MY97 EJ22 MY97 EJ22 9.69 7.5 553 9.69 0.152 11.2 41² 12² 9.6:1 EJ22 9.7:1 stock
MY98 EJ25 MY98 EJ25 9.96 7.9 616 9.96 0.152 11.8 46³ 14² 9.6:1 EJ25 DOHC 9.7:1 stock
MY98 EJ25 MY98 EJ25 9.96 7.9 616 9.96 0.152 11.8 46³ 15§ 9.4:1 EJ25 DOHC stock 2§
MY99 EJ25 MY99 EJ25 9.96 7.9 616 9.96 0.058 4.5 TBD 15§ TBD EJ25 SOHC 10.0:1 stock
MY00 EJ25 MY00 EJ25 9.96 7.9 616 9.96 0.058 4.5 50² 6² 11.2:1 EJ25 SOHC 10.0:1 stock
MY02 EJ20T MY02 EJ20T 9.20 7.5 499 9.20 0.08 5.3 49ß TBD TBD EJ20T 8.0:1 stock
This table summarizes some measurements and calculations for several custom configurations:

Engine Block Cylinder Head Bore¹ (cm) Stroke¹ (cm) Displ (cc) Gasket bore¹ (cm) Gasket Thickness (cm) Quench Vol (cc) Head Vol (cc) Piston Vol (cc) Calc'd CR Notes
MY98 EJ25 MY98 EJ25 9.96 7.9 616 9.96 0.058 4.5 46³ 14* 10.5:1 Thin SOHC head gasket
MY91 EJ22T MY98 EJ25 9.69 7.5 553 9.96 0.152 11.8 46³ 28²¤ 7.4:1 EJ22T/DOHC hybrid
MY91 EJ22T MY98 EJ25 9.69 7.5 553 9.96 0.058 4.5 46³ 28²¤ 8.0:1 Thin SOHC head gasket
MY00 EJ25 MY00 EJ25 9.96 7.9 616 9.96 0.152 11.8 50² 6² 10.1:1 Thick DOHC head gasket
MY91 EJ22T MY00/01 EJ25 9.69 7.5 553 9.96 0.058 4.5 50² 28²¤ 7.7:1 EJ22T/SOHC hybrid
MY91 EJ22T MY00 EJ25 9.69 7.5 553 9.69 0.150 11.2 50² 28²¤ 7.2:1 Thick DOHC head gasket
MY91 EJ22T MY97 EJ22 9.69 7.5 553 9.96 0.058 4.5 41² 28²¤ 8.5:1 Thin SOHC head gasket
¹ From published data.
² From actual measurement. Measurement errors are estimated to be ±0.25cc using a 10cc capacity graduated medical syringe, Pennzoil 10W40 motor oil, and visually sighting the meniscus. Ambient temperature was about 10°C.
³ From Andrew (mcgyver) at NASIOC.
¤ From http://www.sdsefi.com. Notable pages are http://www.sdsefi.com/air17.html and http://www.sdsefi.com/air21.html.
§ From Larry (ImprezaRS dot com) at NASIOC.
ß From Cobb Tuning at NASIOC.
* Estimated from published CR data.

Note that both EJ22 and EJ25 pistons are flush with the deck at TDC, therefore I have assumed that quench volume is determined solely by head gasket thickness. I have done some "what-if" scenarios to see how much of an effect a thicker or thinner head gasket has on the compression ratio to an otherwise stock engine.

I'm not pretending that this is performed using rigorous experimental procedures, but accuracy should be enough to provide ballpark numbers within two significant figures.



What do we learn here?

My 1997 EJ22 measurements and calculations are very close to the official number of 9.7:1 CR, and are certainly well within measurement errors.
The offical word from Subaru on the MY00's compression ratio is 10.0:1. This is somewhat less than measured, so maybe the official number is an "effective compression ratio" as a result of intake valve overlap?
Changing head gasket thickness has a significant effect on compression ratio.
Yellow entry above is a popular hybrid using an EJ22T block with DOHC heads from a MY98 EJ25 2.5RS or MY97-99 2.5GT or Legacy Outback. Note that the use of a thin SOHC head gasket provides a more favourable compression ratio than the original thick DOHC head gasket.
Cyan entry above is another popular hybrid using an EJ22T block with SOHC heads from a newer EJ25. This has been reported to be a highly reliable combination due to a low compression ratio and good-flowing heads.


Last updated: 3 May 2003.

Thanks for reposting this. I was a little bummed when I saw that EJcalcs was down.

Just to clarify, the piston volume (cc) for the ej22t is -28 or 28?
Also is it concluded that the 22t pistons do not protrude past the deck? Unless perhaps resurfaced.. Going through about 30 different forum discussions on the subject and it seems its a 50/50 answer.
I'm in a bind because I need to order head gaskets and I don't want to wait until I take my heads off and measure, and then find I need different gaskets. Thus creating more down time.
I would like to use the .040 gasket to achieve a more desirable CR, but with half the people saying the pistons protrude this seems risky.

When I built my impreza with wiseco pistons they did not protrude from the deck.

I've noted that wiseco piston specs are -20 piston dish volume. So if OEM 22t piston are 28(-28?) Wiseco uses a different spec.

In a different post from mike-tracey he used this CR calculator, here is what I came up with, please correct me if I'm wrong as my #'s seem to vary slightly from this post.
.051 gasket with 0 deck protrusion, -28 PDV, 22t/25d:

.040 gasket, 0 protrusion, -28 PDV, 22t/25d:


Am I doing this correctly? Sorry to impede on your post wtdash, this just seemed a good place to ask without starting another topic.

The oem pistons do protrude on the 22t. I think there was some discussion I made and there were some pretty good points. I don't know if it was in my build thread or in another topic I created.

I hope this helps. I do know that Alphius can give you a direct answer tho.

By the math all phase 1 ej22 pistons, turbo and non-turbo, protrude by 0.3mm. There has been discussion as to whether or not that is true. Every one I've torn down has been slightly out of the deck when measured.

OK. My brain just starts to melt a little bit when I try putting all the math together.
What is the piston to valve clearance? DOHC & SOHC. 25d/22t.
Also in the above pictures my head gasket values are based on wiseco gasket specs. Wtdash has a thicker gaskest and thinner gasket than what I was using, threw me off a little. I always thought the OEM 22t gasket was .051 in. But I see several people stating its thicker.
Outside of larger pistons its seems their is not much other option for upping the compression.